1. Field of the Invention
The invention disclosed in the present specification relates to a thin film transistor using a crystal silicon film and a fabrication method thereof. The invention also relates to a device utilizing such a thin film transistor and a fabricating method thereof.
2. Description of Related Art
Hitherto, there has been known a thin film transistor (hereinafter referred to as TFT) using an amorphous silicon film. It is mainly used for constructing an active matrix circuit of an active matrix liquid crystal display.
However, the TFT using the amorphous silicon film has had a drawback that its operation speed is slow and that a P-channel type TFT cannot be put into practical use. Then, due to such problems, it was unable to fabricate an active matrix liquid crystal display to which a peripheral driving circuit is integrated or to construct various integrated circuits using the TFT.
As a solution of this problem, there has been known a scheme of using a crystal silicon film. The crystal silicon film may be fabricated by heating or by irradiating laser light.
However, the heating method has had a problem that a glass substrate cannot be used because it requires a high temperature process of 900.degree. C. or more. Considering that the main field of application of the TFT is a liquid crystal display, it is a preferential subject to be able to use the glass substrate as the substrate.
Meanwhile, although the method of irradiating laser light can realize a process through which no thermal damage is given to the substrate, it is not satisfactory in terms of the crystal uniformity and reproducibility and of the degree of crystallization.
As one of solutions of such problems, there has been a method of promoting the crystallization by using a predetermined metal element, i.e. the inventive method which the present applicants have proposed. According to this method, a crystal silicon film is obtained by introducing metal element typified by nickel to the amorphous silicon film and by implementing a heat treatment. This method allows the crystal silicon film having good crystallinity to be obtained while implementing the heat treatment below about 600.degree. C. which permits to use the glass substrate.
However, because the nickel element remains within the crystal silicon film, the characteristic of the TFT fabricated by using the crystal silicon film is influenced adversely. In concrete, it causes problems that the characteristic changes as time elapses, thus degrading the reliability.
Accordingly, it is an object of the invention disclosed in the present specification to provide a technology for suppressing the metal element from adversely influencing the characteristic of the TFT fabricated by using the crystal silicon film obtained by utilizing the metal element which promotes the crystallization of silicon.